21720135 has 32 divisors (see below), whose sum is σ = 38741760. Its totient is φ = 10326528.

The previous prime is 21720131. The next prime is 21720137. The reversal of 21720135 is 53102712.

21720135 = T₃₈₀ + T₃₈₁ + ... + T₅₆₉.

It is not a de Polignac number, because 21720135 - 2² = 21720131 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720131) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 2604 + ... + 7086.

It is an arithmetic number, because the mean of its divisors is an integer number (1210680).

Almost surely, 2^21720135 is an apocalyptic number.

21720135 is a deficient number, since it is larger than the sum of its proper divisors (17021625).

21720135 is a wasteful number, since it uses less digits than its factorization.

21720135 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4527.

The product of its (nonzero) digits is 420, while the sum is 21.

The square root of 21720135 is about 4660.4865625812. The cubic root of 21720135 is about 279.0106880452.

Adding to 21720135 its reverse (53102712), we get a palindrome (74822847).

The spelling of 21720135 in words is "twenty-one million, seven hundred twenty thousand, one hundred thirty-five".